Sex or no sex? For water flea, environment determines mating habits

"Perhaps we have more in common with water fleas than we think," says Dr. Gerald LeBlanc (above) of the lessons his research may have for humans.Photo by Daniel Kim

Female water fleas have figured out how to survive - and thrive - without sex. In normal environmental conditions, they reproduce asexually, giving birth to large broods of about 30 females. Those females reproduce asexually and have even more females.

But all good things must come to an end, says Dr. Gerald LeBlanc, professor of environmental and molecular toxicology in the College of Agriculture and Life Sciences. He says that during changes in the water fleas' environment - when conditions become overcrowded, say, or when available amounts of food become depleted - female water fleas begin giving birth to males. These males mate with the previously uncooperative females. The results of this "sex under stress" are eggs that are encased in a protective covering designed to withstand the environmental changes taking place.

Besides giving birth to male offspring during times of environmental change, female water fleas also turn a bright copper color. LeBlanc and colleagues at N.C. State and Harvard Medical School wanted to find out if these responses were related.

The researchers discovered that the responses were indeed regulated by the same signaling pathway in female water fleas. The results are published in the January 2005 edition of The Journal of Experimental Biology.

"We found that when female water fleas receive a hormonal signal to produce males, they also receive a signal to produce more hemoglobin, or oxygen-carrying molecules, which turns females a bright copper color," LeBlanc says.

In the study, funded by the U.S. Environmental Protection Agency, the researchers gave amounts of methyl farnesoate - a hormone recently discovered by LeBlanc's research team to cause female water fleas to give birth to males - to several strains of female water fleas. Some of these strains were characterized as unable to produce males under natural conditions, while the others had the potential of producing males.

"We hypothesized that the non-male producing females would respond to the treatment by producing males and hemoglobin," LeBlanc says, "and that the male-producing females would produce even more males and more hemoglobin."

But that's not what happened. Five out of the six non-male producing strains produced neither males nor hemoglobin. Meanwhile, four of the five male-producing strains responded to the treatment by producing males and hemoglobin.

"What this tells us is that the two processes are regulated by the same hormone and the same signaling process and are linked in a way we don't yet understand," LeBlanc says. "We think the signaling pathway may be compromised in the non-male producing strains of water fleas."

LeBlanc can only speculate about why hemoglobin and male sex determination are co-regulated by environmental and hormonal signals. He says production of males may place mother water fleas under greater metabolic demands resulting in greater oxygen requirements. Or, the cues responsible for the production of more males may also serve as an early-warning system for oxygen depletion in the environment, and hemoglobin may be produced to ensure that mothers survive long enough to procreate. Finally, LeBlanc reasons, male-producing mothers may generate toxic metabolites that can only be assuaged by excess hemoglobin.

Do the couplings of water fleas, or the lack thereof, provide any lessons for humans? LeBlanc isn't sure. But he's aware of a few extreme cases in which exposure to certain chemicals is associated with declines in the proportion of males born. And, he says, according to U.S. Census Bureau data, human male-to-female birth ratios declined between 1972 and 1992.